1. Field of the Invention
The present invention relates to a Cu (copper)-base alloy used as an overlay, and more particularly, to a dispersion strengthened Cu-base alloy for forming an overlay (hardfacing layer) having a superior wear resistance and an improved heat-resistance on a metal substrate.
2. Description of the Related Art
Wear-resistant Cu-base materials include precipitation-strengthened alloys such as beryllium (Be) copper alloys containing about 2% of Be additive and Cu-Ni-Si alloys (e.g., Corson alloy) (cf., e.g., Monma and Sudo: "Constructional Metal Materials and Heat-treatment therefor (revised edition)", Metallurgical Engineering Series No. 1, Japan Institute of Metals, 1980, pp. 20-25), and particle dispersion-strengthened alloys in which hard particles of, e.g., oxide (SiO.sub.2, Cr.sub.2 O.sub.3, BeO, TiO.sub.2, ZrO.sub.2, MgO, MnO, etc.) are dispersed in a Cu-base matrix.
The precipitation-strengthened Cu alloys are subjected to a long time aging treatment, after a solution heat-treatment, to precipitate intermediate phases, intermetallic compounds and the like from the matrix for strengthening. The particle dispersion-strengthened Cu alloys are produced mainly by a sintering processor or an internal oxidation process. In the sintering process, a Cu or Cu alloy powder to made into a matrix is mixed with an oxide powder as a disperse phase, and the mixed powder is compacted and then sintered. In the internal oxidation process, a metal which is more easily oxidized than a Cu or Cu alloy matrix is added to the matrix and is then oxidized by oxygen diffused inwardly under an oxidizing atmosphere at a high temperature, to form oxide phases inside the matrix.
On the other hand, it is well known that a Cu-Pb system alloy, e.g., a Kelmet containing 25 to 35% of Pb, is used as a bearing metal, i.e., a Cu-base wear resistant material. The Cu-Pb system alloy has a structure in which a mixture of a soft Pb phase and a hard Cu phase do not mutually dissolve each other, and the hard Cu sustains a load and the soft Pb forms small hollows as oil reservoirs, and serves as a solid lubricant. (cf., e.g., the above-cited publication, pp. 40-41). Such a Cu-Pb system alloy bearing metal has superior antiseizing properties and has a larger load bearing capacity than that of a white metal, and thus it is suitable for a high speed, high load bearing. Nevertheless, since the Cu-Pb system alloy does not have a sufficient strength, when used for a high speed, high load bearing, it is joined to a backing metal having a suitable strength, to form a bimetal bearing.
The precipitation-strengthened alloys require a long time, high temperature heat-treatment for age precipitating fine particles by diffusion in a solid phase, and this heat-treatment is apt to generate strain in alloy members (parts) and is not suitable for large alloy members (parts). The precipitated particles give a required hardness to the strengthened alloy, but since the precipitated particles are very fine (e.g., several micrometers at most), the strengthened ally does not have a satisfactory wear-resistance, especially a resistance to slide abrasion. A higher resistance to slide abrasion is attained by hard particles having a grain size of 10 to 100 .mu.m and dispersed in the alloy matrix, but it is difficult to precipitate such large size particles in precipitation-strengthened alloys.
Further, one type of the particle dispersion-strengthened alloys made through the internal oxidation process also requires a long time, high temperature heat-treatment for forming the dispersed oxide particles by diffusion in a solid phase, and this heat-treatment also is apt to generate strain in alloy members and is not suitable for large alloy members. Another type of particle dispersion-strengthened alloys obtained by the sintering process contains dispersed oxide particles having a desired grain size but requires a pressing step and a sintering step, by which product members are formed into suitable shapes, and it is difficult to locally form the particle dispersion-strengthened alloy portion in such a product member.
Still further, since the Cu-Pb system bearing alloy is joined to the backing metal of, e.g., steel, for producing a bearing, it is necessary to prepare a bearing supporting member (backing metal) with a high machining accuracy and a bearing member of the Cu-Pb system alloy, respectively, and it is difficult to produce a bearing having a complicated sliding shape. Accordingly, under severe conditions the Cu-Pb system bearing alloy does not have a sufficient wear-resistance, and thus has a lower durability.
The present inventors have studied particle dispersion-strengthened Cu-base alloys for wear-resistant overlays (hardfacing layers) deposited locally or wholly on a metal substrate, and have proposed Cu-base alloys having a structure in which hard particles of silicide and/or boride of Fe-Ni system, Ni-Cr system and the like are dispersed in a Cu-base matrix, for example, a Cu-Ni-Fe-Si-B alloy (see U.S. Pat. No. 4,828,307 based on Japanese Unexamined Patent Publication (Kokai) No. 63-157826), a Cu-Ni-Ti-Si-B alloy (see Japanese Unexamined Patent Publication (Kokai) No. 63-264295), a Cu-Ni-Fe-Cr-Si alloy (see Japanese Unexamined Patent Publication (Kokai) No. 01-111831), and a Cu-Ni-Cr-Si-B alloy (see Japanese Unexamined Patent Publication (Kokai) No. 01-152232), and the wear-resistance, especially a resistance to slide abrasion, of these Cu-base alloys is improved by the dispersed hard particles. Furthermore, the inventors have also proposed a particle dispersion-strengthened Cu-base alloy supplemented with 20 to 40% of Pb (i.e., Cu-Ni-Si-B-Pb alloy, (see Japanese Unexamined Patent Publication (Kokai) No. 01-205043) to give an additional Pb solid lubricity.
Nevertheless, where a particle dispersion-strengthened Cu-base alloy is used under severe conditions, e.g., for a valve seat in an internal combustion engine (e.g., automobile engine), the wear-resistance of the alloys is still not satisfactory, since during the operation of the automobile engine, face portions of the exhaust valves are heated at 700.degree. C. or more, and an exhaust gas passing therethrough has a temperature of 1000.degree. C. or more. When the valve seats come into contact with the heated valve face portion and are exposed to the exhaust gas flow of 1000.degree. C. or more, the surface of the valve seats is also exposed in a very high temperature condition. In particular, the surface temperature of the Cu-base alloy valve sheets may be raised to a temperature close to a melting point thereof, and thus the surface of the Cu-base alloy valve seat is easily adhered to the valve face portion. Once this adhesion occurs, the Cu-base alloy material adhered to the valve face portion comes in contact with the Cu-base alloy surface of the valve seat, with the result that the adhesion becomes remarkable greater to cause considerable wear (abrasion) of the valve. This phenomenon occurs when the Cu-base alloy is used as a wear-resistant material in a sliding or contacting condition in a high temperature.
The above-mentioned conventional and proposed wear-resistant Cu-base alloys mainly utilize a strengthening effect obtained by a second phase precipitation or crystallization, and a solid solution strengthening of Cu rich .alpha. phase primary crystals (as a portion of a matrix) with Ni and the like occurs. Nevertheless, despite this solid solution strengthening, the Cu-base primary crystals are liable to adhesion. In particular, when the Cu-base alloy valve seat comes into contact with a valve face portion made of an alloy which is not easily oxidized, such as an austenitic steel, an Ni-base alloy, and a Co-base alloy, the primary crystals generate adhesion (become attached to the valve).
The Cu-base alloy proposed in the above-mentioned JPP'043 contains hard particles of silicide and boride of nickel, but depending on circumstances the Cu-base alloy may not have a sufficient wear-resistance.